Which Terran sectors you can safely enter is determined by your ATF rank. In plot-enabled games your ATF rank can be increased by doing the Terran Conflict plot. In non-plot games (e.g. Custom) your ATF rank is equal to your Terran rank.

In addition to the sector-level access restrictions, travel within the Earth sector itself is also rigidly controlled. There is a small safe zone between the trans-orbital accelerator and the Torus docking ports. Any ships outside this zone will be destroyed by the Torus guns - which is important to note if you normally use the autopilot to dock, as collision avoidance will send your ship into the danger zone!